Modern high frequency oscilloscope probes are often fabricated as active differential probes. A differential probe has two applied signals whose difference is formed and taken as the desired input signal. With reference now to FIG. 1, one such prior art probing arrangement 1 for a digital oscilloscope 3 from Agilent Technologies has a relatively high frequency amplifier (not shown) that is carried within a housing 2 that connects to the ‘scope 3 through a three or four foot long cable 4 ending in an attachment assembly 5, and that couples to the circuit or device under test through fairly short (four to six inches) lengths of flexible small diameter coaxial transmission lines 6 whose distal ends may have a hand-held ‘browser’ that carries the actual probe tips (the figure doesn't show a browser—keep reading). The browser is a small assembly that includes impedance matching elements or other passive networks interposed between the short flexible transmission lines and the actual probe tips. Browsers are usually fairly small, as oscilloscope probes go, but are nevertheless often too large to be deployed to certain locations, say, one that is between two parallel and closely spaced printed circuit board assemblies. In such a case the hand-held browser is replaced with a solderable probe assembly 7 whose probe tips 8 are actually soldered to the locations bearing the signal to be measured. It dispenses with the handle that makes a browser hand-held, and retains just the bare minimum of ingredients needed to carry the short flexible coaxial transmission lines 6 and the impedance matching elements 9. Such a solderable probe assembly 7 might be on the order of a quarter of an inch long (excluding the probe tips), three sixteenths of an inch wide, and a sixteenth of an inch high. The probe tips 8 are short ( 1/16″) lengths of small diameter (e.g., 0.007″) nickel wire that is solderable, bendable (say, fifteen to thirty times before they break), and replaceable. A roll of replacement wire is supplied with the probe assembly, and a technician uses a microscope and a tiny soldering iron to install new probe tips.
Now refer to FIG. 2. For various well known reasons, it is desirable for there to be a calibration test fixture (10) that receives the solderable probe assembly 7. The probe's output is coupled to a ‘scope input channel in the usual fashion. The calibration test fixture is (either directly, which is actually preferred, or through a cable 11) connected to a front panel calibration test signal (not shown) provided by the ‘scope. Upon instructions to do so, the ‘scope interprets the known calibration test signal as measured by the channel/probe assembly combination, to discover and then institute appropriate post-processing compensation of the measured results, thereby providing more accurate measurements with that particular probe assembly when used with other signals of interest.
One prior art design for a calibration test fixture 10 is embodied by the products E2655A and E2655B from Agilent Technologies. That design involves a terminated strip transmission line 12 formed on a substrate 13 and having a clamp 14 for pinching the probe tips 8 against the transmission line at one of two different possible locations indicated generally by the intersection of index marks (15) printed on the fixture with slots 16 cut into clamp 14. (The slots are there to reduce added inter-probe capacitance present when the probe tips are clamped.) Mechanically, the function of the clamp 14 is reminiscent of the stamped metal clip on an ordinary press-board or plastic clip board used to render a pad of paper portable for writing upon, save that the clamp 14 on the test fixture 10 is considerably smaller and is of an insulating material. The strip transmission line 12 may be terminated directly by a termination 17 attached to a connector 18 that is part of the fixture 10, or, optionally (and which is not shown) terminated instead through a length of coaxial transmission line that connects the downstream end of the fixture 10 to a 50 Ω measurement input of one of the channels possessed by the ‘scope 3.
The design shown in FIG. 2 possesses a number of shortcomings that limit its effectiveness and convenience. Among these are that, since the clamp 14 is a unitary item used for both probe tips, the probe assembly 7 has to be correctly positioned beneath an opened clamp to correctly align both probe tips at the same time before the clamp is released. Misaligned probe tips will introduce a false skew into the measured result, and is a condition that is to be avoided. However, it is difficult to achieve proper alignment with the clamp open and then maintain it while closing the clamp; things tend to wiggle around too much. Furthermore, despite the modest clamping force provided, it is also difficult to coerce the clamped probe tips to change their position beneath the clamp; they might rather bend or break, especially if they have been in use for some time.
Next, the clamping force is not always sufficient to retain a clamped probe. This follows in part from the springs 19 and in part from the nearly central location of the pivot 20 between the springs and the clamping edge 21. However, there is a limit to merely making the springs stiffer; the clamping edge 21 tends to bow when it encounters the probe tips 8, reducing the force applied to the probe tips. A related issue concerning the particular clamp 14 is that its overall length forces the location of the connection between the probe tips 8 and the transmission line 12 to be toward the middle of the test fixture 10. This is not fatal, but in situations involving the highest frequencies and where greatest accuracy is sought, the cable 11 will be absent in favor of directly connecting the fixture to the connector on the ‘scope's front panel, and it would further be desirable if the location along the transmission line 12 touched by the probe tips 8 were as close to the ‘scope as practical. Lastly, experience has shown that the arrangement shown in FIG. 2 lends itself to straightening the probe tips and then clamping them by laying the probe assembly 7 flat against the substrate 13 (and then leaving things that way . . . ). This is a bad habit that is unfortunately encouraged by the design. (It is bad, as the proximity of the probe's grounded shield disturbs the strip transmission line 12.)
It would be desirable if there were an alternate design that both clamped the probe tips with greater force and that allowed individual probe tips to be clamped one at a time, so as to permit an iterative series of adjustments to get both probe tips clamped in their proper locations. Furthermore, that proper location ought to be as close as practical to the driven end of the test fixture.